Color display devices

ABSTRACT

The present invention is directed to color display devices which are capable of displaying multiple color states. The display device comprises a plurality of display cells, wherein each of said display cells is (a) sandwiched between a first layer comprising a common electrode and a second layer comprising a plurality of driving electrodes, wherein at least one of the driving electrodes is a designated electrode, (b) filled with an electrophoretic fluid comprising at least two types of pigment particles dispersed in a solvent or solvent mixture, and (c) capable of displaying at least four color states. The display device may also comprise hiding layers or a brightness enhancement structure on the viewing side.

This application is a divisional application of U.S. application Ser.No. 12/416,827, filed Apr. 1, 2009, which claims priority to U.S.Provisional Application No. 61/042,180, filed Apr. 3, 2008; the contentsof the above-identified applications are incorporated herein byreference in their entirety.

FIELD OF THE INVENTION

The present invention is directed to display devices in which eachdisplay cell is capable of displaying multiple color states. The displaydevice may also comprise blocking layers such as black matrix layers ora brightness enhancement structure on the viewing side.

BACKGROUND OF THE INVENTION

Prior technologies relating to reflective color displays have, ingeneral, used red/green/blue (RGB) colors. In the RGB color displays,each pixel is broken down into three or four sub-pixels and eachsub-pixel has a red filter, blue filter, green filter or no filter overa black and white reflective medium. By selectively turning sub-pixelson or off, a full color spectrum may be achieved. For example, the whitestate is achieved by turning on all three RGB sub-pixels. Because eachof the RGB sub-pixels occupies ⅓ of the white spectrum, the maximumreflectivity of each sub-pixel is about ⅓ of the reflectivity of thewhite color. The maximum brightness of each of the RGB colors thereforeis ⅓ of the white color since the sub-pixels of the other two colors areturned off. In practice, a reflective display actually achievessubstantially less than this theoretical brightness.

Over the years, many pixel architectures have evolved, including addingtwo green sub-pixels or a white sub-pixel to gain some brightness at theexpense of color saturation. However, in general, the RGB color systemshows very limited whiteness and poor color brightness.

U.S. Pat. No. 7,046,228 discloses an electrophoretic display devicehaving a dual switching mode which allows the charged pigment particlesin a display cell to move in either the vertical (up/down) direction orthe planar (left/right) direction. U.S. patent application Ser. No.12/370,485 discloses an alternative multiple color display. In bothcases, display cells are filled with a dyed fluid having white particlesdispersed therein and each of the display cells is capable of displayingthree color states, i.e., the color of the charged pigment particles,the color of the dielectric solvent or solvent mixture or the backgroundcolor of the display cell. These two types of the display can provideRGB color states which are as bright as those achieved by the RGB typedisplay. They, in addition, can also provide non-compromised white andblack color states which could not be achieved by the RGB type display.

SUMMARY OF THE INVENTION

The present invention is directed to further alternative designs ofcolor display devices. The color display of the present invention allowseach display cell to display at least four different color states, whichenables a significant increase in color brightness (from 50% to morethan 3 times in color brightness compared to the RGB type display),while maintaining the non-compromised black and white color states.

The first aspect of the invention is directed to a display device whichcomprises a plurality of display cells, wherein each of said displaycells is

(a) sandwiched between a first layer comprising a common electrode and asecond layer comprising a plurality of driving electrodes, wherein atleast one of the driving electrodes is the designated electrode,

(b) filled with an electrophoretic fluid comprising at least two typesof pigment particles dispersed in a solvent or solvent mixture, and

(c) capable of displaying at least four color states.

The display device may further comprise blocking layers which are on theviewing side of the display device and are positioned corresponding tothe designated electrode(s) in order to hide the pigment particles whichgather at the designated electrode(s). The blocking layer may be a blackmatrix layer or a micro-structure or micro-reflector of a brightnessenhancement structure.

In one embodiment, the electrophoretic fluid comprises two types ofpigment particles dispersed in a solvent or solvent mixture. The twotypes of pigment particles may carry opposite charge polarities or carrythe same charge polarities but are of different electrophoreticmobilities. In the two types of pigment particles, one may be white andthe other may be red, green or blue. The solvent or solvent mixture inthe electrophoretic fluid may be red, green or blue.

In another embodiment, the electrophoretic fluid comprises three typesof pigment particles dispersed in a solvent or solvent mixture. Thethree types of pigment particles may carry different charge polaritiesor different levels of the same charge polarity. In the three types ofpigment particles, one may be white and the other two may be red andgreen, red and blue or green and blue. The solvent or solvent mixture inthe electrophoretic fluid may be red, green or blue.

In a further embodiment, the driving electrodes are not aligned with theboundary of the display cell.

In yet a further embodiment, the pigment particles are driven to thedesignated electrode(s) all at once. Alternatively, the pigmentparticles are driven to the designated electrode(s) in steps.

In yet a further embodiment, the driving electrodes on the second layerare a grid of at least 2×2.

In yet a further embodiment, the first layer is on the viewing side.Alternatively, the second layer is on the viewing side.

In yet a further embodiment, the designated electrodes are nottransparent. For example, they may be opaque.

In yet a further embodiment, each pixel in the display device may havetwo, three or four sub-pixels.

BRIEF DISCUSSION OF THE DRAWINGS

FIG. 1 a depicts a cross-section view of a display cell of a colordisplay device of the present invention.

FIG. 1 b depicts a top view of the layer comprising driving electrodes.

FIGS. 1 c and 1 d depict alternative designs of the layer comprisingdriving electrodes.

FIG. 2 illustrates how the charged pigment particles may move to thedesignated electrodes in steps.

FIG. 3 depicts the driving electrodes not aligned with the boundaries ofthe display cells.

FIGS. 4 a-4 e illustrate how four different color states may bedisplayed.

FIGS. 5 a-5 e illustrate how five different color states may bedisplayed.

FIGS. 6 a-6 d illustrate different configurations of a pixel.

FIG. 7 a depicts a color display of the present invention with blackmatrix layers on the viewing side of the display.

FIG. 7 b depicts a color display of the present invention with abrightness enhancement structure on the viewing side of the display.

FIGS. 8 a-8 g show examples of how a brightness enhancement structuremay be fabricated.

FIG. 9 a is a three dimensional view of the brightness enhancementstructure with micro-structures or micro-reflectors.

FIG. 9 b is an alternative design of a brightness enhancement structure.

FIGS. 10 a and 10 b show how the black matrix layers and themicro-structure or micro-reflector are aligned with designatedelectrodes, respectively.

FIGS. 11 a-11 d depict an alternative design of color display devices ofthe present invention.

FIGS. 12 a-12 e depict a further alternative design of color displaydevices of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

I. Configuration of a Display Device

FIG. 1 a depicts a cross-section view of a display cell of a colordisplay device of the present invention. The display cell (100) issandwiched between a first layer (101) and a second layer (102). Thefirst layer comprises a common electrode (103). The second layercomprises more than one driving electrode (104 ax, 104 ay, 104 az, 104bx, 104 by, 104 bz, 104 cx, 104 cy and 104 cz). However, in thecross-section view, only the driving electrodes 104 cx, 104 cy and 104cz are shown.

In one embodiment, each display cell, as shown in FIG. 1 a, represents asingle sub-pixel or pixel.

FIG. 1 b depicts the top view of the layer comprising driving electrodesof the display cell of FIG. 1 a. As shown, the second layer (102)comprises 3×3 driving electrodes, denoted as 104 ax, 104 ay, 104 az, 104bx, 104 by, 104 bz, 104 cx, 104 cy and 104 cz. While only a 3×3 grid isshown, the second layer may comprise any grid which is at least 2×2. Thesize of the driving electrodes may vary, depending on the size of thedisplay cell. There is a gap between the driving electrodes. In otherwords, the driving electrodes are not in contact with each other.

In the context of the present invention, the driving electrode(s)intended for the charged pigment particles to gather in order to behidden from the viewer is/are referred to as the “designatedelectrode(s)”.

In FIGS. 1 a and 1 b, if driving electrode 104 cy is the designatedelectrode, then there is a blocking layer (108) on top of the displaycell on the viewing side in a position corresponding to the designatedelectrode 104 cy, so that any particles gather at the designatedelectrode (104 cy) will be hidden from the viewer.

The multiple driving electrodes within a display cell allow theparticles to migrate to one or more designated electrodes or to spreadover all the driving electrodes.

The 9 driving electrodes in FIG. 1 b are shown to have the same shapeand size. It is understood that the shapes and sizes of the drivingelectrodes in the same display device may vary, as long as they servethe desired functions.

Optionally, there is a background layer (not shown), which may be abovethe second layer (102) or below the second layer (102). Alternatively,the second layer may serve as a background layer. The background layerpreferably is black.

The common electrode (103) is usually a transparent electrode layer(e.g., ITO), spreading over the entire top of the display device. Thedriving electrodes (104 s) may be active matrix electrodes which aredescribed in U.S. Pat. No. 7,046,228, the content of which isincorporated herein by reference in its entirety. It is noted that thescope of the present invention is not limited to the driving electrodesbeing active matrix electrodes. The scope of the present applicationencompasses other types of electrode addressing as long as theelectrodes serve the desired functions.

It is also shown in FIG. 1 b that the 9 driving electrodes are alignedwith the boundary of the display cell (100). However, for this type ofcolor display, this feature is optional. Details of an un-alignedconfiguration are given below.

While the first layer (101) is shown in FIG. 1 a as the viewing side, itis also possible for the second layer (102) to be on the viewing side.This is illustrated as alternative designs discussed in Section VIbelow.

The display cells are filled with an electrophoretic fluid whichcomprises at least two types of pigment particles dispersed in a solventor solvent mixture. The different types of pigment particles may carrycharges of opposite polarity.

It is also possible to have two types of pigment particles carrying thesame charge polarity but with different electrophoretic mobilities, ifthe mobility of one pigment is substantially different from that of theother. The mobilities of the pigment particles may arise from differentparticle sizes, particle charges or particle shapes. Coating or chemicaltreatment of the surfaces of the pigment particles can also be used toadjust the electrophoretic mobility of the pigment particles.

Alternative designs of the second layer (102) are shown in FIGS. 1 c and1 d. In FIG. 1 c, the center electrode “D” is the designated electrodewhereas the non-designated driving electrode “N-D” surrounds thedesignated electrode D. In FIG. 1 d, a designated electrode (D) issandwiched between two non-designated electrodes (N-D). Thesealternative designs have the advantage that there are fewer addressingpoints that are needed, thus reducing the complexity of the electricalcircuit design. It is also noted that there may be different numbers ofthe designated and non-designated electrodes and the designated andnon-designated electrodes may be of any shapes; but the non-designatedelectrode(s) must be larger in total area than the designatedelectrode(s).

For the alternative designs as shown, the designated electrode(s) andthe non-designated electrode(s) must be aligned with the boundary of thedisplay cell.

In the context of the present invention, the migration of the chargedpigment particles to the designated electrode(s) may occur all at once,that is, the voltages of the common and driving electrodes are set atsuch to cause the charged pigment particles to migrate to be at or nearthe designated electrode(s) all at once. Alternatively, the migrationmay take place in steps. As shown in FIG. 2, the voltages of drivingelectrodes are set at such to cause the charged pigment particles tomove from one driving electrode to an adjacent driving electrode onestep at a time and eventually to the designated electrode(s). Thisdriving method may prevent the charged pigment particles from beingtrapped at the center of one large driving electrode even though thelarge driving electrode has the same polarity as the pigment particles.

Another one of the advantages of the color display of the presentinvention is that the driving electrodes do not have to be aligned withthe boundary of the display cell. As shown in FIG. 3, the display cells(represented by the dotted lines) and the driving electrodes(represented by the solid lines) are not aligned. In this case, thecharged pigment particles may still be driven to show the desired colorstates. To accomplish this, a scanning method or similar approaches maybe used to first determine which driving electrodes address whichdisplay cell. Those driving electrodes (shaded in FIG. 3) at the edgesof the display cells may never be used or may be used to drive onlypartial areas of the driving electrodes. However, in the latter case,cross-talk may occur.

II. Two Types of Particle System

FIGS. 4 a-4 d illustrate an example of how different color states may bedisplayed. There are two types of pigment particles in theelectrophoretic fluid filled in the display cell. The two types ofpigment particles are of the white and blue colors respectively, andthey move independently from each other because they carry charges ofopposite polarities. It is assumed that the white pigment particles arenegatively charged and the blue pigment particles are positivelycharged. It is also assumed that the two types of pigment particles aredispersed in a solvent of green color.

While only three driving electrodes are shown, it is understood thatthere may be more driving electrodes and in any case, it is assumed thatonly driving electrode 404 cy is the designated electrode.

When a negative voltage potential is imposed on the common electrode(403) and a positive voltage potential is imposed on the drivingelectrodes (404), the positively charged blue particles are drawn to thecommon electrode (403) and the negatively charged white particles to thedriving electrodes (404). As a result, a blue color is seen at theviewing side, as shown in FIG. 4 a.

In FIG. 4 b, when a negative voltage potential is imposed on the drivingelectrodes (404) and a positive voltage potential is imposed on thecommon electrode (403), the positively charged blue particles are drawnto the driving electrodes (404) and the negatively charged whiteparticles to the common electrode (403). As a result, a white color isseen at the viewing side.

FIG. 4 c shows a scenario in which a positive voltage potential isimposed on the designated electrode (404 cy) and a negative voltagepotential is imposed on the non-designated driving electrodes (404 cxand 404 cz). The common electrode (403) is held at ground. In this case,the negatively charged white particles are moved to be at or near thedesignated electrode (404 cy) which is hidden from the viewer by theblocking layer (408) and the positively charged blue particles are onthe non-designated driving electrodes visible from the viewing side. Theblue particles reflect the incoming light with only the spectrum of theblue color. Since the green color of the solvent does not transmit asignificant amount of the blue light, a black color is perceived andseen from the viewing side.

While only one driving electrode (404 cy) is shown to be a designedelectrode, in practice, the number of such designated electrodes may bemore than one. In other words, there may be one or more such designatedelectrodes. The one designated electrode or multiple designatedelectrodes may be any of the driving electrodes, location wise.

In the context of the present invention, the driving electrode(s)intended for the charged pigment particles to gather in order to behidden from the viewer is/are referred to as the “designatedelectrode(s)” and the driving electrodes not intended for the chargedpigment particles to be hidden from the viewer are referred to as the“non-designated electrodes”. For a designated electrode, there is ablocking layer on top of the display cell on the viewing side in aposition corresponding to the designated electrode, so that theparticles gather at the designated electrode are hidden from the viewingside.

FIG. 4 d (a top view) illustrates an alternative way of achieving theblack color state. The driving electrodes may appear in black if thereis a black background layer above or below the second layer.Alternatively, the driving electrodes may appear in black if the secondlayer itself is black. A black background layer on the drivingelectrodes may provide an alternative black color state as shown in FIG.4 d. In this case, there are two designated electrodes. The whitepigment particles gather at one of the designated electrodes whereas theblue pigment particles gather at another designated electrode. The blueand white particles are hidden from the viewer by blocking layers. Thereare substantially no pigment particles gathering at the non-designatedelectrodes. Usually a layer of the black color can absorb more lightthan the combination of the fluid and particles. As a result, a highquality black state is seen from the viewing side.

In the alternative approach as shown in FIG. 4 d, there may be blockinglayers to hide both the blue and white particles, as illustrated.However the blocking layer for the blue particles in fact is optional.In other words, only the blocking layer is always needed to hide thewhite particles.

FIG. 4 e shows another scenario in which a positive voltage potential isimposed on the non-designated driving electrodes (404 cx and 404 cz) anda negative voltage potential is imposed on the designated electrode (404cy). The common electrode (403) is held at ground. In this case, thenegatively charged white particles move to be at or near thenon-designated driving electrodes and act as a reflector. The colorstate of the display cell, seen from the viewing side is green.

The two types of particle system as shown in FIGS. 4 a-4 e can beimplemented with a solvent of any one of the primary colors and pigmentparticles of any other primary colors. Options may also exist fortransparent subtractive color particles and solvents.

The two types of particles system can display four color states in asingle sub-pixel, namely, white, black, green and blue, as shown inFIGS. 4 a-4 e.

III. Three Types of Particle System

FIGS. 5 a-5 e illustrate another example of how different color statesmay be displayed, according to the present invention. There are threetypes of pigment particles in the electrophoretic fluid filled in thedisplay cell. For illustration purpose, the three types of pigmentparticles are of the red, white and blue colors respectively, and theymove independently from each other because they carry different levelsof charge polarities. It is assumed that the white particles arepositively charged, while the blue particles carry a weaker negativepolarity and red particles carry a stronger negative polarity. It isalso assumed that the three types of pigment particles are dispersed ina solvent of green color.

While only five driving electrodes are shown, it is understood thatthere may be more driving electrodes and in any case, it is assumed thatonly driving electrodes 504 cy and 504 cu are the designated electrodes.

FIG. 5 a shows the white state at the viewing side by imposing anegative voltage potential to the common electrode (503), causing thepositively charged white pigment particles to move to be at or near thecommon electrode (503), while the red and blue particles are at or nearthe driving electrodes (504). The white particles directly reflect theincoming light and show the viewer a non-compromised white state.

FIG. 5 b shows the black state from the viewing side by moving the whiteparticles to the designated driving electrodes (504 cy and 504 cu). Thewhite particles are not visible from the viewing side due to thepresence of a blocking layer (508) with registration to the designatedelectrodes. The blue and red particles are at or near the non-designateddriving electrodes (504 cx, 504 cz and 504 cv). Since the solvent is ofthe green color which does not transmit much red or blue color light, ablack color is perceived and seen from the viewing side.

FIG. 5 c shows the blue state from the viewing side by moving the bluepigment particles to be at or near the common electrode (503). In thiscase, a positive voltage potential is first imposed on the commonelectrode (503) and a negative voltage potential is imposed on selecteddriving electrodes (504) which are not necessarily the designatedelectrodes. For illustration purposed, the selected driving electrodesare marked B, C and D. As a result, the blue and red pigment particlesmove to be at or near the common electrode (503) whereas the whiteparticles move to be at or near those selected driving electrodes (B, Cand D).

In a second step, a strong positive voltage potential is imposed on theremaining driving electrodes A and E while a weaker positive voltagepotential is imposed on the common electrode (503). Since the blueparticles carry a weaker negative polarity than the red particles, theywould move slower compared to the red particles under such an electricfield. Accordingly, the blue particles will remain at or near the commonelectrode (503) while the red particles move to be at or near drivingelectrodes A and E.

FIG. 5 d shows the red state from the viewing side by moving the redparticles to be at or near the common electrode (503). In this case, apositive voltage potential is first imposed on selected drivingelectrodes A, D and E and a negative voltage potential is imposed on theremaining driving electrodes B and C. The selected driving electrodesare not necessarily the designated electrodes. The common electrode(503) is held at ground. As a result, the blue and red pigment particlesmove to be at or near the driving electrodes A, D and E whereas thewhite particles move to be at or near the driving electrodes B and C.

In a second step, a strong positive voltage potential is imposed on thecommon electrode (503) while a weaker positive voltage potential isimposed on driving electrodes A, D and E. Since the red particles carrya stronger negative polarity than the blue particles, they would movefaster to be at or near the common electrode (503) while the blueparticles would remain at or near driving electrodes A, D and E.

FIG. 5 e shows the green state from the viewing side by moving the whiteparticles to the non-designated electrodes (504 cx, 504 cz and 504 cv)and the red and blue particles to the designated electrodes (504 cy and504 cu), respectively. Since the red and blue particles are hidden fromthe viewing side, only the green color is seen from the viewing side.

The three types of particles system can display five color states,namely, white, black, red, green and blue in a single pixel, as shown inFIGS. 5 a-5 e.

IV. Configurations of Pixels

Each pixel in a display device of the present invention may be dividedinto a number of sub-pixels. The following examples are given based onthe two types of particle system illustrated in Section II above.

FIG. 6 a shows an example of three sub-pixels (X, Y and Z) in a pixel.In sub-pixel X, the display cell is filled with an electrophoretic fluidcomprising green and white pigment particles dispersed in a solvent ofblue color. In sub-pixel Y, the display cell is filled with anelectrophoretic fluid comprising blue and white pigment particlesdispersed in a solvent of red color. In sub-pixel Z, the display cell isfilled with an electrophoretic fluid comprising red and white pigmentparticles dispersed in a solvent of green color. In the system of FIG. 6a, a high quality white color may be achieved by turning all threesub-pixels to the white state and a high quality black color may beachieved by turning all three sub-pixels to the black state. All threesub-pixels may achieve the white state according to the example of FIG.4 b and they may achieve the black state according to the example ofFIG. 4 c or 4 d.

For a green color, sub-pixel Z (with the green solvent) may be switchedto the green state by spreading the white pigment particles across thenon-designated driving electrodes according to the example of FIG. 4 eand sub-pixel X (with the green particles) is switched to the greenstate by moving the green particles to be at or near the commonelectrode according to the example of FIG. 4 a. Sub-pixel Y, in thiscase, may be switched to the black state according to the example ofFIG. 4 c or 4 d. The viewer, on the viewing side, cannot resolve thethree sub-pixels with naked eyes and as a result, will see a green color(i.e., two out of three sub-pixels being green). The green colorprovided by the present system therefore is twice as intense as thegreen color provided by the RGB system, thus significantly improving thebrightness of the color states.

Alternatively, sub-pixel Y may be switched to the white state if ahigher level of brightness with less color saturation is desired. Thisless saturated green option for sub-pixel Y provides even morebrightness.

A red or blue color may be similarly achieved by the pixel having threesub-pixels X, Y and Z, as shown in FIG. 6 a.

FIG. 6 b is an example in which one pixel consists of only twosub-pixels (X and Y). In sub-pixel X, the display cell is filled with anelectrophoretic fluid comprising red and white pigment particlesdispersed in a solvent of green color. In sub-pixel Y, the display cellis filled with an electrophoretic fluid comprising blue and whitepigment particles dispersed in a solvent of green color. In the systemof FIG. 6 b, a high quality white color may be achieved by turning bothsub-pixels X and Y to the white state and a high quality black color maybe achieved by turning both sub-pixels X and Y to the black state. Bothsub-pixels may achieve the white state according to the example of FIG.4 b and the black state according to the example of FIG. 4 c or 4 d.

For a green color, both sub-pixels X and Y may be switched to the greenstate by spreading the white pigment particles across the non-designateddriving electrodes and it provides a 100% effective area of reflectanceof the green color. It is also capable of providing a red state withsub-pixel X being switched to red and sub-pixel Y being switched toblack. Similarly a blue color may be achieved by switching sub-pixel Xto black and sub-pixel Y to blue. The red and blue colors achieved bythe present system have a higher quality than those achieved by the RGBcolor system, because in the present system one half of the pixel showsthe intended color whereas in the RGB system only one third of the pixelshows the intended color.

FIGS. 6 c and 6 d provide additional examples. Each pixel in the twofigures consists of four sub-pixels.

In FIG. 6 c, in sub-pixel X, the display cell is filled with anelectrophoretic fluid comprising blue and white particles dispersed in asolvent of red color. In sub-pixel Y, the display cell is filled with anelectrophoretic fluid comprising red and white particles dispersed in asolvent of green color. In sub-pixel Z, the display cell is filled withan electrophoretic fluid comprising blue and white particles dispersedin a solvent of green color. In sub-pixel U, the display cell is filledwith an electrophoretic fluid comprising green and white particlesdispersed in a solvent of blue color.

In FIG. 6 d, in sub-pixel X, the display cell is filled with anelectrophoretic fluid comprising blue and white particles dispersed in asolvent of green color. In sub-pixel Y, the display cell is filled withan electrophoretic fluid comprising red and white particles dispersed ina solvent of green color. In sub-pixel Z, the display cell is filledwith an electrophoretic fluid comprising green and white particlesdispersed in a solvent of red color. In sub-pixel U, the display cell isfilled with an electrophoretic fluid comprising green and whiteparticles dispersed in a solvent of blue color.

There may be many other pixel designs that could be developed to providethe optimal output. In all cases, however, much superior colorperformance is achieved compared to the RGB system and it is achievedwith no compromise in black and white color states.

The three types of particle system as illustrated in Section III aboveprovide further improved qualities of red, green, blue, white and blackcolor states without compromise because each color has fully (100%)utilized its area of reflection.

V. Blocking Layers

(a) Black Matrix Layers

In one embodiment of the present invention, the blocking layer is ablack matrix layer (705 a) on the viewing side of the color display, asshown in FIG. 7 a. The display cell (700) is sandwiched between a firstlayer (701) comprising a common electrode (703) and a second layer (702)comprising driving electrodes (704). The designated electrode (704 cy)is shown to be located underneath the black matrix layer (705 a). As aresult, the charged pigment particles gathered at or near the designatedelectrode (704 cy) will not be seen, from the viewing side.

The black matrix layer may be applied by a method such as printing,stamping, photolithography, vapor deposition or sputtering with a shadowmask. The optical density of the black matrix may be higher than 0.5,preferably higher than 1. Depending on the material of the black matrixlayer and the process used to dispose the black matrix, the thickness ofthe black matrix may vary from 0.005 μm to 50 μm, preferably from 0.01μm to 20 μm.

In one embodiment, a thin layer of black coating or ink may betransferred onto the surface where the black matrix layers will appear,by an offset rubber roller or stamp.

In another embodiment, a photosensitive black coating may be coated ontothe surface where the black matrix layers will appear and exposedthrough a photomask. The photosensitive black coating may be apositively-working or negatively-working resist. When apositively-working resist is used, the photomask have openingscorresponding to the areas not intended to be covered by the blackmatrix layer. In this case, the photosensitive black coating in theareas not intended to be covered by the black matrix layer (exposed) isremoved by a developer after exposure. If a negatively-working resist isused, the photomask should have openings corresponding to the areasintended to be covered by the black matrix layer. In this case, thephotosensitive black coating in the areas not intended to be covered bythe black matrix layer (unexposed) is removed by a developer afterexposure. The solvent(s) used to apply the black coating and thedeveloper(s) for removing the coating should be carefully selected sothat they do not attack the layer of the display and other structuralelements.

Alternatively, a colorless photosensitive ink-receptive layer may beapplied onto the surface where the black matrix layers will appear,followed by exposure through a photomask. If a positively-workingphotosensitive latent ink-receptive layer is used, the photomask shouldhave openings corresponding to the areas intended to be covered by theblack matrix layer. In this case, after exposure, the exposed areasbecome ink-receptive or tacky and a black matrix may be formed on theexposed areas after a black ink or toner is applied onto those areas.Alternatively, a negatively-working photosensitive ink-receptive layermay be used. In this case, the photomask should have openingscorresponding to the areas not intended to be covered by the blackmatrix layer and after exposure, the exposed areas (which are notintended to be covered by the black matrix layer) are hardened while ablack matrix layer may be formed on the unexposed areas (which areintended to be covered by the black matrix layer) after a black ink ortoner is applied onto those areas. The black matrix may be post cured byheat or flood exposure to improve the film integrity andphysical-mechanical properties.

In another embodiment, the black matrix may be applied by printing suchas screen printing or offset printing, particularly waterless offsetprinting.

FIG. 10 a shows how the black matrix layers (1003 a) are aligned withthe designated electrodes (1002) to allow the designated electrodes tobe hidden from the viewer. To achieve the “hiding” effect, the width(w1) of the black matrix layer (1003 a) must be at least equal to thewidth (w2) of the designated electrode(s) (1002). It is desirable thatthe width (w1) of the black matrix layers is slightly greater than thewidth (w2) of the designated electrode(s) to prevent loss of contrastwhen viewed at an angle.

In another embodiment, the black matrix layers are not aligned with thedesignated electrodes. In this case, the width of the black matrixlayers is significantly greater than the width of the designatedelectrodes, so that the designated electrodes may be hidden from theincoming light.

(b) Bright Enhancement Structure

In another embodiment, the blocking layer may be a brightnessenhancement structure (708) comprising micro-structures ormicro-reflectors (705 b) on the viewing side of the display device, asshown in FIG. 7 b. The display cell (700) is sandwiched between a firstlayer (701) comprising a common electrode (703) and a second layer (702)comprising driving electrodes (704). The designated electrode (704 cy)is shown to be located underneath the micro-structure or micro-reflector(705 b). As a result, the charged pigment particles gathered at or nearthe designated electrode (704 cy) will not be seen, from the viewingside, using the micro-structures or micro-reflectors to block thedesignated electrodes. In the context of the present invention, thecavity 705 b is referred to as the “micro-microstructure” or“micro-reflector”.

In the context of the present invention, the surface of themicro-structures is uncoated. The term “micro-reflector” refers to amicro-structure the surface of which is coated with a metal layer.Details of the brightness enhancement structure and how it is fabricatedare given below.

The brightness enhancement structure may be fabricated in many differentways. The details of the brightness enhancement structure are disclosedin U.S. patent application Ser. Nos. 12/323,300, 12/323,315, 12/370,485and 12/397,917, the contents of which are incorporated herein byreference in their entirety.

In one embodiment, the brightness enhancement structure may befabricated separately and then laminated over the viewing side of thedisplay device. For example, the brightness enhancement structure may befabricated by embossing as shown in FIG. 8 a. The embossing process iscarried out at a temperature higher than the glass transitiontemperature of the embossable composition (800) coated on a substratelayer (801). The embossing is usually accomplished by a male mold whichmay be in the form of a roller, plate or belt. The embossablecomposition may comprise a thermoplastic, thermoset or a precursorthereof. More specifically, the embossable composition may comprisemultifunctional acrylate or methacrylate, multifunctional vinylether,multifunctional epoxide or an oligomer or polymer thereof. The glasstransition temperatures (or Tg) for this class of materials usuallyrange from about −70° C. to about 150° C., preferably from about −20° C.to about 50° C. The embossing process is typically carried out at atemperature higher than the Tg. A heated male mold or a heated housingsubstrate against which the mold presses may be used to control theembossing temperature and pressure. The male mold is usually formed of ametal such as nickel.

As shown in FIG. 8 a, the mold creates the prism-like brightnessenhancement micro-structures (803) and is released during or after theembossable composition is hardened. The hardening of the embossablecomposition may be accomplished by cooling, solvent evaporation,cross-linking by radiation, heat or moisture. In the context of thepresent invention, the cavity (803) is called a micro-structure.

The refraction index of the material for forming the brightnessenhancement structure is preferably greater than about 1.4, morepreferably between about 1.5 and about 1.7.

FIG. 9 a is a three-dimensional view of the brightness enhancementstructure with brightness enhancement micro-structures (903)corresponding to those (803) as shown in FIG. 8 a.

FIG. 9 b is an alternative design of the brightness enhancementstructure. The micro-structures or micro-reflectors (903) are in acontinuous form. The continuous micro-structures or micro-reflectors areparticularly suitable for a display device with a second layercomprising driving electrodes as shown in FIG. 1 d.

The brightness enhancement structure may be used as is or further coatedwith a metal layer.

The metal layer (807) is then deposited over the surface (806) of themicro-structures (803) as shown in FIG. 8 b. Suitable metals for thisstep may include, but are not limited to, aluminum, copper, zinc, tin,molybdenum, nickel, chromium, silver, gold, iron, indium, thallium,titanium, tantalum, tungsten, rhodium, palladium, platinum and cobalt.Aluminum is usually preferred. The metal material must be reflective,and it may be deposited on the surface (806) of the micro-structures,using a variety of techniques such as sputtering, evaporation, rolltransfer coating, electroless plating or the like.

In order to facilitate formation of the metal layer only on the intendedsurface (i.e., the surface 806 of the micro-structures), a strippablemasking layer may be coated before metal deposition, over the surface onwhich the metal layer is not to be deposited. As shown in FIG. 8 c, astrippable masking layer (804) is coated onto the surface (805) betweenthe openings of the micro-structures. The strippable masking layer isnot coated on the surface (806) of the micro-structures.

The coating of the strippable masking layer may be accomplished by aprinting technique, such as flexographic printing, driographic printing,electrophotographic printing, lithographic printing, gravure printing,thermal printing, inkjet printing or screen printing. The coating mayalso be accomplished by a transfer-coating technique involving the useof a release layer. The strippable masking layer preferably has athickness in the range of about 0.01 to about 20 microns, morepreferably about 1 to about 10 microns.

For ease of stripping, the layer is preferably formed from awater-soluble or water-dispersible material. Organic materials may alsobe used. For example, the strippable masking layer may be formed from are-dispersible particulate material. The advantage of the re-dispersibleparticulate material is that the coated layer may be easily removedwithout using a solubility enhancer. The term “re-dispersibleparticulate” is derived from the observation that the presence ofparticles in the material in a significant quantity will not decreasethe stripping ability of a dried coating and, on the contrary, theirpresence actually enhances the stripping speed of the coated layer.

The re-dispersible particulate consists of particles that are surfacetreated to be hydrophilic through anionic, cationic or non-ionicfunctionalities. Their sizes are in microns, preferably in the range ofabout 0.1 to about 15 um and more preferably in the range of about 0.3to about 8 um. Particles in these size ranges have been found to createproper surface roughness on a coated layer having a thickness of <15 um.The re-dispersible particulate may have a surface area in the range ofabout 50 to about 500 m²/g, preferably in the range of about 200 toabout 400 m²/g. The interior of the re-dispersible particulate may alsobe modified to have a pore volume in the range of about 0.3 to about 3.0ml/g, preferably in the range of about 0.7 to about 2.0 ml/g.

Commercially available re-dispersible particulates may include, but arenot limited to, micronized silica particles, such as those of theSylojet series or Syloid series from Grace Davison, Columbia, Md., USA.

Non-porous nano sized water re-dispersible colloid silica particles,such as LUDOX AM can also be used together with the micron sizedparticles to enhance both the surface hardness and stripping rate of thecoated layer.

Other organic and inorganic particles, with sufficient hydrophilicitythrough surface treatment, may also be suitable. The surfacemodification can be achieved by inorganic and organic surfacemodification. The surface treatment provides the dispensability of theparticles in water and the re-wetability in the coated layer.

In FIG. 8 d, a metal layer (807) is shown to be deposited over theentire surface, including the surface (806) of the micro-structures andthe surface (805) between the micro-structures. Suitable metal materialsare those as described above. The metal material must be reflective andmay be deposited by a variety of techniques previously described.

FIG. 8 e shows the structure after removal of the strippable maskinglayer (804) with the metal layer 807 coated thereon. This step may becarried out with an aqueous or non-aqueous solvent such as water, MEK,acetone, ethanol or isopropanol or the like, depending on the materialused for the strippable masking layer. The strippable masking layer mayalso be removed by mechanical means, such as brushing, using a spraynozzle or peeling it off with an adhesive layer. While removing thestrippable masking layer (804), the metal layer (807) deposited on thestrippable masking layer is also removed, leaving the metal layer (807)only on the surface (806) of the micro-structures.

FIGS. 8 f and 8 g depict an alternative process for depositing the metallayer. In FIG. 8 f, a metal layer (807) is deposited over the entiresurface first, including both the surface (806) of the micro-structuresand the surface (805) between the micro-structures. FIG. 8 g shows thatthe film of micro-structures deposited with a metal layer (807) islaminated with a film (817) coated with an adhesive layer (816). Themetal layer (807) on top of the surface (805) may be conveniently peeledoff when the micro-structure film is delaminated (separated) from theadhesive layer (816) coated film (817). The thickness of the adhesivelayer (816) on the adhesive coated film is preferably in the range ofabout 1 to about 50 um and more preferably in the range of about 2 toabout 10 um.

The brightness enhancement structure comprising micro-structures(uncoated with a metal layer) or micro-reflectors (coated with a metallayer) is then laminated over a layer of display cells as describedabove.

FIG. 10 b shows how the micro-structures or micro-reflectors (1003 b)are aligned with the designated electrodes to allow the designatedelectrodes to be hidden from the viewer. To achieve the “hiding” effect,the width (w3) of the base (1001) of the micro-structure ormicro-reflector (1003 b) must be at least equal to the width (w4) of thedesignated electrode(s) (1002). It is acceptable if the width (w3) ofthe base of the micro-structure or micro-reflector is slightly greaterthan the width (w4) of the designated electrode(s).

The brightness enhancement structure (1000) is formed of a highrefractive index material, and the tilted surface (1004) is reflectiveto the incoming light source due to the total internal reflection (TIR)phenomenon. The area underneath the micro-structure or micro-reflectorwill not receive any light. During the state while the color of thebackground layer is shown, the charged pigment particles migrate tothose designated electrodes underneath the micro-structures ormicro-reflectors, thus avoiding light leakage.

In another embodiment, the micro-structures or micro-reflectors are notaligned with the designated electrodes. In this case, the width of thebase of the microstructures or micro-reflectors is significantly greaterthan the width of the designated electrodes, so that the designatedelectrodes may be hidden from the incoming light.

VI. Alternative Designs

FIGS. 11 a-11 d depict an alternative design of color display devices.In this alternative design, the color display device does not requireblack matrix layers or a brightness enhancement structure.

The display cell (1100), in this design, is also sandwiched between afirst layer (1101) and a second layer (1102). The first layer comprisesa common electrode (1103). The second layer comprises more than onedriving electrodes.

While only three driving electrodes are shown, it is understood thatthere may be more driving electrodes and in any case, it is assumed thatonly driving electrode 1104 cy is the designated electrode.

As shown the color display device is viewed from the driving electrodeside (i.e., the second layer) instead of the common electrode side(i.e., the first layer).

The driving electrode layer also comprises multiple driving electrodesas shown in FIG. 1 b. While only a 3×3 grid is shown in FIG. 1 b, thedriving electrodes may be of a grid which is at least 2×2. The multipledriving electrodes within a display cell allow the particles to migrateto one or more designated electrodes or evenly spread over all thedriving electrodes.

In this design, the non-designated driving electrodes are transparentand the designated electrode(s) is/are non-transparent. The designatedelectrodes may be opaque.

There are two types of pigment particles in the electrophoretic fluidfilled in the display cell. The two types of pigment particles are ofthe white and blue colors respectively, and they move independently fromeach other because they carry charges of opposite polarities. It isassumed that the white pigment particles are negatively charged and theblue pigment particles are positively charged. It is also assumed thatthe two types of pigment particles are dispersed in a solvent of greencolor.

In FIG. 11 a, a negative voltage potential is imposed on the commonelectrode (1103) and a positive voltage potential is imposed on thedriving electrodes (1104), the negatively charged white particles aredrawn to the driving electrodes (1104) and the positively charged blueparticles to the common electrode (1103). As a result, a white color isseen at the viewing side, as shown in FIG. 11 a.

In FIG. 11 b, when a negative voltage potential is imposed on thedriving electrodes (1104) and a positive voltage potential is imposed onthe common electrode (1103), the positively charged blue particles aredrawn to the driving electrodes (1104) and the negatively charged whiteparticles to the common electrode (1103). As a result, a blue color isseen at the viewing side.

FIG. 11 c shows a scenario in which a negative voltage potential isimposed on the designated electrode (1104 cy) and a positive voltagepotential is imposed on the common electrode (1103). The non-designateddriving electrodes (1104 cx and 1104 cz) are held at ground. In thiscase, the negatively charged white particles move to be at or near thecommon electrode (1103) while the positively charged blue pigmentparticles move to be at or near the designated electrode (1104 cy). Theblue particles are hidden from the viewer because the designatedelectrode is non-transparent. As a result, a green color is seen fromthe viewing side.

While only one driving electrode (1104 cy) is shown to be a designedelectrode, in practice, the number of such designated electrodes may bemore than one. In other words, there may be one or more such designatedelectrodes. The one designated electrode or multiple designatedelectrodes may be any of the driving electrodes, location wise.

FIG. 11 d shows another scenario in which a negative voltage potentialis imposed on the common electrode (1103) and a positive voltagepotential is imposed on the designated electrode (1104 cy). Thenon-designated driving electrodes (1104 cx and 1104 cz) are held atground. In this case, the negatively charged white particles move to beat or near the designated driving electrode (1104 cy) and the positivelycharged blue particles move to be at or near the common electrode. Sincethe green color of the solvent does not transmit a significant amount ofthe blue light, a black color is perceived and seen from the viewingside.

The two types of particles system can display four color states, namely,white, black, green and blue, as exemplified in FIGS. 11 a-11 d.

FIGS. 12 a-12 e illustrate an alternative design with three types ofparticles. The particle and solvent system of FIGS. 12 a-12 e is thesame as that of FIGS. 5 a-5 e. Namely, white, red and blue particles aredispersed in a solvent of green color. It is assumed that the whiteparticles are positively charged, while the blue particles carry aweaker negative polarity and red particles carry a stronger negativepolarity.

In this alternative design, the color display device does not requireblack matrix layers or a brightness enhancement structure. The displaycell (1200) is sandwiched between a first layer (1201) and a secondlayer (1202). The first layer comprises a common electrode (1203). Thesecond layer comprises more than one driving electrodes.

While only five driving electrodes are shown, it is understood thatthere may be more driving electrodes and in any case, it is assumed thatonly driving electrodes 1204 cy and 1204 cu are the designatedelectrodes.

As shown the color display device is viewed from the driving electrodeside (i.e., the second layer) instead of the common electrode side(i.e., the first layer). The non-designated driving electrodes aretransparent and the designated electrode(s) is/are non-transparent. Thedesignated electrodes may be opaque.

In FIG. 12 a, a negative voltage potential is imposed on the drivingelectrodes (1204 s) and a positive voltage potential is imposed on thecommon electrode (1203). As a result, the positively charged whitepigment particles move to be at or near the driving electrodes. A whitecolor therefore is seen from the viewing side.

In FIG. 12 b, a negative voltage potential is imposed on the designatedelectrodes 1204 cy and 1204 cu and a positive voltage potential isimposed on the common electrode (1203). The non-designated drivingelectrodes (1204 cx, 1204 cz and 1204 cv) are held at ground. As aresult, the white particles move to be at or near the designatedelectrodes while the red and blue particles move to be at or near thecommon electrodes. Since the solvent is of the green color which doesnot transmit much red or blue color light reflected back from the red orblue particles, a black color is perceived and seen from the viewingside.

In FIG. 12 c, the blue particles move to be at or near thenon-designated driving electrodes (1204 cx, 1204 cz and 1204 cv). Inthis case, a positive voltage potential is first imposed on thenon-designated driving electrodes (1204 cx, 1204 cz and 1204 cv) and anegative voltage potential is imposed on the designated drivingelectrodes (1204 cy and 1204 cu). The common electrode (1203) is held atground. As a result, the blue and red pigment particles move to be at ornear the non-designated electrodes (1204 cx, 1204 cz and 1204 cv)whereas the positively white particles move to be at or near thedesignated electrodes (1204 cy and 1204 cu).

In a second step, a strong positive voltage potential is imposed on thecommon electrode (1203) while a weaker positive voltage potential isimposed on the non-designated driving electrodes (1204 cx, 1204 cz and1204 cv). Since the red particles carry a stronger negative polaritythan the blue particles, they would move faster to be at or near thecommon electrode while the blue particles remain at or near thenon-designated electrodes. As a result, a blue color is seen from theviewing side.

In FIG. 12 d, the red particles move to be at or near the non-designateddriving electrodes. In this case, a positive voltage potential is firstimposed on the common electrode (1203) and a negative voltage potentialis imposed on the designated driving electrodes (1204 cy and 1204 cu).As a result, the blue and red pigment particles move to be at or nearthe common electrode (1203) whereas the positively charged whiteparticles move to be at or near the designated electrodes (1204 cy and1204 cu).

In a second step, a strong positive voltage potential is imposed on thenon-designated electrodes (1204 cx, 1204 cz and 1204 cv) while a weakerpositive voltage potential is imposed on the common electrode (1203).Since the blue particles carry a weaker negative polarity than the redparticles, they would remain at the common electrode while the redparticles move to be at or near the non-designated electrodes (1204 cx,1204 cz and 1204 cv). As a result, a red color is seen from the viewingside.

In FIG. 12 e, shows the green state from the viewing side by moving thewhite particles to the common electrode (1203) and the red and blueparticles to the designated electrodes (1204 cy and 1204 cu). Since thered and blue particles are hidden from the viewing side, only the greencolor is seen from the viewing side.

The three types of particles system can display five color states,namely, white, black, red, green and blue, as shown in FIGS. 12 a-12 e.

While the present invention has been described with reference to thespecific embodiments thereof, it should be understood by those skilledin the art that various changes may be made and equivalents may besubstituted without departing from the true spirit and scope of theinvention. In addition, many modifications may be made to adapt aparticular situation, materials, compositions, processes, process stepor steps, to the objective, spirit and scope of the present invention.All such modifications are intended to be within the scope of the claimsappended hereto.

What is claimed is:
 1. A display device comprising a plurality ofdisplay cells, wherein each of said display cells is (a) filled with anelectrophoretic fluid comprising a first type of pigment particles of afirst color and a second type of pigment particles of a second color,both of which are dispersed in a solvent or solvent mixture of a thirdcolor, (b) sandwiched between a first layer comprising only one commonelectrode and a second layer comprising a plurality of drivingelectrodes, among which at least one driving electrode is anon-transparent designated driving electrode and the remaining drivingelectrodes are transparent non-designated driving electrodes, whereinthe two types of pigment particles are capable of being driven to thefirst layer, to the second layer, and to the at least onenon-transparent designated driving electrode, and (c) operable fordisplaying at least four color states when the display cell is viewedthrough the second layer, wherein the pigment particles which gatheredat the at least one non-transparent designated driving electrode arehidden and are not visible from the viewing side.
 2. The display deviceof claim 1, wherein said non-transparent designated driving electrode isopaque.
 3. The display device of claim 1, wherein the two types ofpigment particles carry opposite charge polarities.
 4. The displaydevice of claim 1, wherein the two types of pigment particles are ofdifferent electrophoretic mobilities.
 5. The display device of claim 1,wherein the first color is white and the second color is red, green orblue.
 6. The display device of claim 1, wherein the third color is red,green or blue.
 7. The display device of claim 1, wherein saidelectrophoretic fluid further comprises a third type of pigmentparticles dispersed in the solvent or solvent mixture and the threetypes of pigment particles carry different levels of charge polarity. 8.The display device of claim 1, wherein said electrophoretic fluidfurther comprises a third type of pigment particles dispersed in thesolvent or solvent mixture and one type of the pigment particles iswhite and the other two types of the pigment particles are red andgreen, red and blue, or green and blue.
 9. The display device of claim8, wherein the third color is red, green or blue.
 10. The display deviceof claim 1, wherein said driving electrodes are a grid of at least 2×2.11. The display device of claim 1, wherein each display cell is asubpixel, and two, three or four sub-pixels form a pixel.
 12. Thedisplay device of claim 1, wherein said display cells are microcups ormicrocapsules.